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If I understand what you want to do, just use the "magic wand" tool, BUT be sure to un-check the "contiguous" option.   If you do this and have a reasonable setting for the range, one click will select every pixel in the image that has the same tonality (brightness).  See attached for an example.

To put each of the 8 (or so) levels onto it's own layer, just hit cntrl-C, cntrl-V after you select each tonal range and you will get what you want.   Obviously, to be able to clearly see what is on each layer, when you have completed the 8 select-cut-paste cycles, stick a layer with a neutral color between the original image and the stack of new layers that you just created.  This entire procedure is very, very fast.  It probably took me 30 seconds to work through the image I posted.